A. Field of the Invention
The present invention relates to recombinant DNA technology. It is especially useful in allowing the safe introduction of foreign DNA into eukaryotic cells.
B. Description of the Art
There has been much interest in introducing foreign DNA into eukaryotic cells. One reason for this interest is that some genetically caused diseases may be curable by introducing the foreign DNA into the cells, and allowing the foreign DNA to express a protein that the genetically defective cell cannot express. Another reason for this interest is that certain eukaryotic cells may prove to be the most suitable hosts for the production of certain eukaryotic proteins. Yet another reason is that this may permit farm animals to be genetically improved.
A promising approach for achieving the introduction of foreign DNA into eukaryotic cells was disclosed in an article by K. Shimotohno et al., 26 Cell 67-77 (1981). (The disclosure of this article and of all other articles and patents cited in this application are incorporated by reference herein as if fully set forth). This approach used plasmids with retrovirus portions for growing up a stock of virus, and then used the progeny virus as a carrier for introducing foreign DNA into the vertebrate cell genome.
These "retrovirus vectors" were designed so as to be defective with respect to the expression of certain retrovirus structural genes. Thus, by themselves they were replication defective. They could, however, be replicated (packaged) to grow up a stock of the virus by using DNA virus of helper cells.
One type of helper cell contained helper virus DNA that supplied the missing viral gene products. The helper virus DNA itself was encapsidation-minus, so its RNA was not packaged. By using this helper cell, pure virus stock could be obtained which did not contain helper virus. See generally U.S. Pat. No. 4,650,764, issued Mar. 17, 1987 to Temin et al.
One theoretical problem with such systems is that because of their design it is conceivable that an endogenous retrovirus (e.g. one already in a host) could act as a pseudo helper virus. Thus, after infecting a host with the virus, the virus could replicate in the host (and cause disease). Another problem is that a promoter sequence present in the provirus that is formed after infection of the host might trigger cellular gene sequences (leading to side effects such as perhaps cancer).
Thus, it can be seen that a need exists for a retrovirus vector that can be replicated in a helper cell to produce progeny virus that will infect an eukaryotic host, but is designed so that the resulting provirus will be promoter deficient. Ideally, such a system should also be designed to reduce the risks of recombination and permit efficient expression of the foreign gene of interest.